On the importance of lensing for galaxy clustering in photometric and spectroscopic surveys

Abstract

We study the importance of gravitational lensing in modelling the number counts of galaxies for the first time in spectroscopic surveys. We confirm previous results for photometric surveys, showing that lensing cannot be neglected in a survey like LSST since it would infer a significant shift of standard cosmological parameters. For a spectroscopic survey like SKA2, we find that neglecting lensing in the monopole, quadrupole and hexadecapole of the correlation function can also induce an important shift of cosmological parameters. For ΛCDM parameters, the shift is moderate, of the order of 0.6σ or less. However, for a model-independent analysis, that measures the growth rate of structure in each redshift bins, neglecting lensing introduces a shift of up to 2.3σ at high redshift. Since the growth rate is directly used to test the theory of gravity, such a strong shift would wrongly be interpreted as the breakdown of General Relativity. This shows the importance of including lensing in the analysis of future surveys. For a survey like DESI, we find on the other hand that lensing is not important, mainly due to the value of the magnification bias parameter of DESI, s(z), which strongly reduces the lensing contribution at high redshift. This result relies on our theoretical modelling of s(z) in DESI and should therefore be confirmed with measurements of s(z) in simulations. We also propose a way of improving the analysis of spectroscopic surveys, by including the cross-correlations between different redshift bins (which is neglected in spectroscopic surveys) from the spectroscopic survey or from a different photometric sample. We show that including the cross-correlations in the SKA2 analysis does not improve the constraints. On the other hand replacing the cross-correlations from SKA2 by cross-correlations measured with LSST improves the constraints by 10% to 20%. The marginal improvement is mainly due to the density correlations between nearby bins and, therefore, does not strongly depend on our knowledge of the magnification bias. Interestingly, for standard cosmological parameter estimation, the photometric survey LSST in its 12 redshift bin configuration and the spectroscopic SKA2 survey are highly complementary, since they are affected by different degeneracies between parameters: LSST yields the tightest constraints on Ωcdm, h and n s, while SKA2 better constrains Ωbaryon, A s and the bias.

Anti-symmetric clustering signals in the observed power spectrum

“…Although not considered a GR effects per se, peculiar velocities perturb the assumed volume and change the observed clustering. RSD is the largest such correction but others may also be relevant and detectable, such as lensing magnification [11] and the Doppler effect [12]. Peculiar velocities should alter the observed redshift solely from the Doppler effect, but also change their apparent size [13][14][15].…”

Multi-wavelength spectroscopic probes: prospects for primordial non-Gaussianity and relativistic effects

“…We identified which pairs of correlations are most sensitive to the lensing and Doppler effects, using a signal-to-noise estimator S AB in (4.2) for angular power spectra (similar to the estimator used in the 2-point correlation function by [76]). The results are summarised in Figure 5.…”

The Kaiser-Rocket effect: three decades and counting

“…Here we estimate the bias on recovered cosmological parameters of a f NL ΛCDM-model -a ΛCDM-model with an extra parameter for the amplitude of a (small) primordial local non-Gaussianity-in the presence of unsubtracted Kaiser rocket effect. We adopt the forecasting approach described in [52][53][54][55][56][57][58][59][60][61][62][63][64][65][66].…”

“…For a Gaussian-distributed observable Ψ, one can estimate the systematic shift syst = ϑ (I) − ϑ (C) of the best-fitting parameters ϑ bf,I and ϑ bf,C from a measurement where Ψ is modelled incorrectly or correctly, respectively, using [52][53][54][55][56][57][58][59][60][61][62][63][64][65][66] syst = F −1…”